The Firebrand Philosopher : DIDEROT: A Critical Biography, <i> By P.N. Furbank (Alfred A. Knopf: $30; 424 pp.) </i>

Didier Diderot, a master cutler of Langres in Champagne, had two sons. Denis, the elder, was noisy and rebellious. Didier, the younger, was quiet and devout; eventually he became a priest.

“Alas,” old Didier explained after both were grown, “one will certainly be a saint and I am very much afraid that the other may be damned. But I cannot live with the saint, and I greatly enjoy the time I spend with the damned one.”

So did the numerous friends of Denis Diderot, philosopher, man of letters, epitome of the Enlightenment and creator with D’Alembert of the great encyclopedia that was its charter. So did posterity, which found in novels and essays unpublishable in Diderot’s lifetime a variety and paradox that make him, more than Voltaire or Rousseau, a contemporary right up into our own day. And so will readers of P. N. Furbank’s rich, high-spirited, restless and thus utterly Diderotesque critical biography.

The French Enlightenment preceded the Revolution, and its philosophes saw themselves as overturning all the intellectual and social fixities that had accrued in 18th-Century France. They had accrued in layers, and often contradictory ones; for example, Cartesian thought, revolutionary in its own day, had become one more element in a variegated and unstable theological orthodoxy.

Revolution settles into its own orthodoxy. For many Enlightenment figures--even Rousseau, or especially Rousseau--it was a matter of substituting one assertiveness for another. D’Alembert, for example, witness to various half-hearted official suppressions of the encyclopedia, sought to have his own critics suppressed. It was part of the simmering in that extraordinary time that Malesherbes, royal minister in charge of publications and thus, technically, the chief censor, was the most passionate of believers in free speech.

Furbank sets out the simmering background and sets the figure of Diderot, bold and timid, impetuous and cautious, profoundly individual and passionately sociable, moving through it. His book is a critical biography but the author, biographer of E. M. Forster, possesses an almost novelistic vividness. In fact, more than in the analysis of individual works--sometimes tedious or fuzzy--his achievement is to connect these works to a central passion and to use that passion to make a portrait.

The passion was motion, a continual, never-resting motion away from orthodoxy; and then, away from wherever that first motion had landed you. Diderot could not abide constraint, neither the constraint of what existed nor that of the ideas that he and others devised to overthrow it.

It was skepticism, but not the Voltairean kind we associate with the 18th Century, nor even the muscular kind--highly creative, of course; it created the modern world--of the 19th. It was not muscle but pure nerve. It questioned not one particular reality but each successive one. It questioned its way, in some respects, to the abyss of 20th-Century modernism and the stylized post-modernist arches that don’t bridge it (you can’t walk on them) but comment on it.

Diderot no doubt felt constrained in Langres. As a teen-ager, he tried to run away to Paris; paradoxically, he wanted to join the Jesuits. After he had insubordinately announced his decision to marry, his father had him locked up in a monastery, from which he was permitted to escape by climbing through a window. Later, as a firebrand philosophe , he was imprisoned, not very stringently, in the fortress of Vincennes. High-born friends interceded, and after promising to publish nothing that would offend religion, he was released. He had groveled, but he had to. Movement was his vital principle.

We see him moving constantly. For the encyclopedia, he would tackle anything. Among the A’s, he did articles on steel ( acier ), authority, Arabs and agriculture. In his passion to discover how things work, he visited craftsmen in workshops. Queen Marie-Antoinette read a clandestine edition and was delighted to be able to learn just how rouge was made. Her husband vaguely wondered why he was forbidding the publication of such a useful work.

Diderot was constantly going to dine or stay at country weekends. He would intervene with his high-placed connections to help all manner of people; like Voltaire, he was admired in all the courts of Europe. He was warm, voluble, paradoxical and funny, and he couldn’t sit still. He poked or grabbed his interlocutors; Catherine the Great complained she was black and blue after a three-hour session.

As we read of these things, Furbank walks about quite as actively and occasionally takes our arm to conduct us through Diderot’s writing. With Locke, Hume and his fellow- philosophes , he was a materialist, arguing that we reach reality and form our ideas through our senses, and that we have no innate ideas either from God or any other higher source. And we suspect that, arguments aside, it was a philosophy he needed.

It allowed him to move and to move on, to visit artisans and artists, to evade any thought that was prescribed to him, even his own. It allowed him freedom from reality; a blind man, having different senses, has a different reality, he speculated. It allowed him to be free even from reason, whose chains can be as binding as any others.

Writing for the theater, he objected to being addressed by the actors. They should address each other and leave the audience free to eavesdrop, he argued. His plays were mediocre but his theories influenced such playwrights as Gotthold Lessing. In his art criticism, he refused to let the picture address him frontally. He wrote as if he had stepped into the scene and were poking about inside it and making discoveries. He loved the variousness of painting; he distrusted the marble unity of sculpture. “Marble doesn’t laugh,” he wrote.

In his three great philosophic novels--”Rameau’s Nephew,” “Jacques the Fatalist” and “D’Alembert’s Dream,” all published posthumously, he refuses to impose a single world of illusion on his readers, as 19th-Century novels would do. He kept breaking the narrative, interrupting his characters, letting them answer back, inventing readers and letting the readers interrupt. We are with Laurence Sterne (whom he admired) in one sense; in another, we are up in the 20th Century.

Furbank would not have succeeded so well in this lavish and stimulating portrait had he not taken account of Diderot’s many contradictions. His thought was more radical than Rousseau’s, yet he was personally and politically cautious. His philosophy of contradictions had its own contradictions; his awareness of them is one of the things that makes both him and his writing so appealing.

Despite his theoretical distrust of sculpture, Diderot had a passion for statues. He would walk in the gardens of the chateaux he visited and converse with them. He would imagine that humans are destined, in a way, by practicing virtue and exercising their talents, to fashion statues of themselves. “Virtue for him was an effortful form of art,” Furbank writes of this hugely spontaneous man.

There is another thing. For a materialist who longs for immortality, a statue--not the object itself but what it suggests--is the closest thing he can allow to an angel touching him on the shoulder.